Many communication devices operate on battery power. The radiotelephones in use today are almost all portable devices utilizing rechargeable batteries that need periodic recharging. The size of the battery and the power efficiency of the communication device determine the amount of talk time available to a user. As a convenience to a user, it has always been desirable to extend the life of the battery to provide longer talk times. However, even with current devices, communications are occasionally interrupted, requiring the recharging of the battery or replacement with a freshly recharged battery. Therefore, it has been desirable to extend the battery life on a communication device.
In most cases, the largest drain on battery power is the transmitter of the communication device, followed by the receiver. The transmitter, or more specifically the power amplifier of the transmitter, in a communication device has typically been configured so that it only draws power when the user wishes to transmit. As the user controls the transmission time, and the level of power transmitted is controlled by standard, the current drain drawn on a battery from the transmitter circuit has already been limited to those times and amounts that are strictly necessary for communication. However, the power used by the receiver is less controlled. For example, the receiver must be ready to receive an incoming call that may occur at any time. For this to happen, the receiver must be on, drawing power, and ready to receive.
Receiver operation is different in different operating systems. For example, the Global System for Mobile (GSM) communication system requires mobile radiotelephones to monitor neighboring cell frequencies for the possibility of reselection in case of dropped service. The intent is that the mobile radiotelephone decodes and synchronizes to neighboring cell carriers, so that the mobile radiotelephone has these carriers from which to choose at the time it determines that a reselection to a different carrier is necessary. The mechanism in the GSM specifications by which the mobile radiotelephone chooses the neighboring cell carriers is the relative signal level (RSS) on the neighboring cell carriers (e.g. broadcast control channels or BCCH). The receiver operates to monitor these channels and keep a list of the strongest available channels. This monitoring is time-gated such that the receiver is powered down (sleeps) during those known times when transmissions cannot be received (i.e. no information is being transmitted). In other words, the receiver only operates at particular time when transmissions can exist to be received. This is referred to as discontinuous reception (DRX) operation. The goal of DRX operation is to reduce the on-time of the radio by powering-off portions of the radio, such as the receiver, during idle (sleep) periods.
In DRX operation, when the radiotelephone is in an idle or sleep mode (i.e., not engaged in a call), the radiotelephone will periodically power its receiver up to monitor any available control channels of neighboring cells and to look for paging activity indicating incoming messages (pages) for the device. In addition, the device monitors the strength of the control channels to determine when a handoff between cells is warranted. In the idle state, the radiotelephone wakes up only during slots preassigned by the radiotelephone system to determine if there are any detectable broadcast control channels (BCCH) and to decode the base station identity code (BSIC) data. The available broadcast control channels to detect are predefined in a BCCH Allocation (BA) list, which is periodically updated, indicating the possible neighboring cells available to the device. However, this periodic type of receiver operation still wastes current since detection is done even when there is no channel signal present or there is no imminent reselection.
The GSM specification 05.08, “Digital Cellular Telecommunications System (Phase 2+); Radio Subsystem Link Control”, (European Telecommunications Standards Institute (ETSI), European Standard (Telecommunications Series), version 6.7.2, section 6.6.1, “Monitoring of Received Signal Level and BCCH data”, regulates when Receive Signal Strength (RSS) monitoring of neighboring channels must be done. However, this specification does allow the suspension of scanning neighboring cell signal strength depending upon the signal quality of the calling channel. Therefore, one solution to reduce receiver operation and current drain has been to eliminate wakeups and channel signal strength measurements if either the BA list is empty, if an RF signal on any channel of the BA list does not exist, or if an RF signal exists but is of a non-valid communication system signal (i.e. not suitable for reselection). The specification also allows skipping the monitor for channel signal strengths to every other second or fourth defined wakeup period. Therefore, another solution to the problem is to simply provide a fixed reduction percentage of channel signal detections performed. However, in both of these cases the monitoring reduction is driven by an instruction from the network and neither technique takes advantage of the signal quality detection, other than if a signal is present or absent.
Accordingly, there is a need for a method and apparatus for reducing current drain in a communication device using received control channel signal quality information. There is a further need to reduce the current drain by the receiver without the need for network instructions. It would also be of benefit to provide these advantages without additional hardware, which would increase the cost of the communication device.